There is a vent in the gas cap. It allows air in to prevent a vacuum as gas is used up but will not allow air out. It acts like a check valve. If you are not experiencing the engine running poorly because of a vacuum buildup in the tank causing restricted gas flow then all is well.

many thanks Sagebrush. I've found that the engine floods when the cap is fitted so I'm putting it in an ultrasonic cleaner just in case the breather is blocked. (I also found a couple of tips on this forum relating to the same problem).
I'll keep you informed when I've found the cause/problem/ or sorted.

What is the normal fuel pump pressure of an 1100, maybe 2 psi? The float valves if working correctly would certainly hold this pressure and most likely a good deal more.

The theory of pressure build up in tank blowing past float valves can be checked easy enough with a fuel pressure gauge attached in line. I will guess that it would take 5-10 psi to do this and if that much pressure were in the tank would blow the fuel cap off like a rocket when removed.

Local and need repair help with your 1500, Valkyrie or ST please click contact
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Thanks CT1500, heres the update: I removed the carbs and checked the float needles, and yes one of them was not seating properly. So a good clean with some metal polish has put that right. I also ultrasonically cleaned the tank filler cap to ensure the small vents where clear, this also has rectified the pressure in the tank. It still builds up slightly, but nowhere near what it was doing. While I was in the carbs, I set the float hight and all seems fine with that too. The one thing that I did find was that the accelerator pump diaphragm had a tiny tear in it so I'm waiting on a replacement now. Hopefully all will be good once it's put back together. Thanks for all the people that took the time to answer me, I certainly helped.

If your mixture screws have flags or stops on them limiting adjustment (new vehicle emission required when built) now is the time to disable them to allow full adjustment travel for future settings/adjustment.

Local and need repair help with your 1500, Valkyrie or ST please click contact
Nothing leaves my shop till its' perfect
This is what I do